The spinal column is a flexible chain of closely linked vertebral bodies. In a normal human spine there are seven cervical, twelve thoracic and five lumbar vertebral bodies. Below the lumbar vertebrae are the sacrum and coccyx. Each individual vertebra has an outer shell of hard, dense bone. Inside the vertebra is a honeycomb of cancellous bone containing red bone marrow. All of the red blood cells and many of the white blood cells are generated inside this cancellous bone, where the blood cells mature before being released into the blood circulation.
The spinal disc serves as a cushion between the vertebral bodies to permit controlled motion. A healthy disc consists of three components: a gelatinous inner core called the nucleus pulposus; a series of overlapping and laminated plies of tough fibrous rings called the annulus fibrosus; and two superior and inferior thin cartilage layers, connecting the disc to the thin cortical bone of the vertebral bodies, called the endplates.
The spinal disc may be displaced or damaged due to trauma or disease, such as a herniation or degenerative disc disease.
A herniated disc may bulge out and compress itself onto a nerve, resulting in lower leg pain, loss of muscle control, or paralysis. To treat a herniated disc, the offending nucleus portions are generally removed surgically.
Disc degeneration gradually reduces disc height, forcing the annulus to buckle, tear or separate radially or circumferentially, and causing persistent and disabling back pain. Degenerative disc disease is generally treated presently by surgically removing the nucleus and fusing the adjacent vertebral bodies to stabilize the joint.
In either case, whether removing a portion of the nucleus or all of the nucleus, these procedures ultimately place greater stress on adjacent discs to compensate for the lack of motion, which may cause premature degeneration of those adjacent discs.
Modern trends in surgery include the restoration of bodily function and form (i.e., repair) of anatomical structures through the use of minimally invasive surgical techniques. The ability to surgically repair damaged tissues or joints, creating as few and as small incisions as possible, produces less trauma, less pain and better clinical outcomes in general.
An emerging technique to treat degenerative disc disease is to replace the degenerated nucleus with a prosthetic nucleus in an attempt to restore function, versus fusion which severely limits the function of the spine. Since a degenerated nucleus can be removed using relatively small diameter instruments (e.g. 5 mm or less), this approach is more conducive to minimally invasive techniques.
A deficiency of current minimally invasive surgical techniques to replace the nucleus is the difficulty in determining whether enough space in the disc has been created to properly fit an implant. Creating the proper dimension cavity may be particularly important when implanting a device that expands, such as with a hydrogel implant. If the cavity created is larger than the implant, unintended implant movement or instability can occur. If the cavity created is smaller than the implant, an implant either may not fit, may not be positioned correctly or an expandable device may not achieve its proper functional shape.